Electric wave amplifier



ELECTRIC WAVE AMPLIFIER Filed May 19, 1932 ,NTOR c. B. A [KEN BUM EVEN NUMBER OF 5 TA 6E5 ATTORNEY Patented Feb. 27, 1934 UNITED STATES. PATENT OFFICE ELECTRIC WAVE AWLIFIER Charles E.- Aiken, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 19, 1982. Serial No. 612,208

11 Claims. (02. 179-471) tion. The reduction in the modulation is eifected by a reverse feedback from the output to the input of a single stage or in the case of a plurality of stages the feedback may be from the output of one stage to the input of a preceding stage.

One application of the invention is to radio broadcast receiving and distributing systems for hotels or apartment houses, in which an untuned amplifier is used to amplify simultaneously all of the program bands coming in on the receiving aerial so as to increase their volume before they are distributed to the individual receivers. Such an amplifier must have low modulation to prevent beats or interference between the various received signals.

The feedback or degenerating voltage is obtained from an inductance-resistance combination, so arranged as to offer substantially no impedance to direct current or low frequency currents, but to present a relatively high impedance to frequencies lying in the received band. Any even order'harmonics, sum and difference frequencies or other even order products of the received frequencies which are generated in the amplifier and. flow through the common leg in which the inductance-resistance combination is located, set up a voltage drop across this combination. This drop is introduced into the same or a preceding stage of the amplifier in such a phase as to cause degeneration and consequent reduction of the even order modulation products. The magnitude of the feedback may be regulated by adjusting the magnitude of the resistance in the inductance-resistance combination or by adlusting the loss in the circuit connecting this combination to the earlier point in the amplifier stage.

A very important feature of the amplifier is the arrangement which makes possible the reduction, or complete elimination, of the most serious phantom signals which occur in any given installation. This is done by tuning a radio receiver connected to the output of the amplifier to the frequency of the phantom and adjusting the resistive impedance until the phantom is reduced by a maximum amount or is entirely eliminated.

These phantoms as noted are due to even hermonies, generated in the amplifier, of strong local stations or to sum or difference frequencies of two strong stations. Cross-talk of this type is much more objectionable than that due to odd order modulation and the ability to eliminate it represents a great advance in centralized radio systems.

Not only may even order products which are lically generated be reduced but if a received station radiates an even order harmonic the fre quency of which lies within the received band then the feedback arrangements described may be employed to balance the received harmonic against that generated locally with a resulting large reduction in the amplifier output of this harmonic.

The invention will be better understood from the following description, together with the attached drawing forming a part thereof, in which Fig. 1 illustrates the invention as applied to a single stage of amplification;

Figs. 2 and 3 illustrate two forms the inven tion may take when applied to amplifiers having an odd number of stages; and

Fig. 4 illustrates the invention as applied to an amplifier having an even number of stages.

Referring now to Fig. 1, two space discharge devices 2 and 3 are arranged in push-pull or balanced relation. Associated with devices 2 and 3 are input transformer 4 and output transformer 5. Across each part of the secondary winding of transformer 4 there is shunted a resistor to provide the proper impedance relation between the devices 2 and 3 and the line to which the input transformer is connected.

Resistanoes 6 and 7 shunted by condensers are connected in the anode circuits of devices 2 and 3, respectively, between the corresponding cathodes and ground to provide negative grid bias from plate circuit drop of potential in a manner well known in the art. These resistors may be replaced by batteries or other sources of direct current having low capacity to ground.

The anode current source, indicated at 3+, is connected to the anodes through a choke coil 8 and each half of the primary winding of output transformer 5. Stopping condenser 9 prevents the grounding of the anode current source at 10. As is the common practice in amplifiers,

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the negative terminsl of the anode current source is grounded.

.in lnductsnce ii and variable resistsnce 12 are connected in porsllel between the junction of resistances c and 7 and ground to. it loud 13 connects the grids of devices 2 and 3 also to ground 10 through the respective halves of the secondary winding of input transformer d and the shunting resistances. The inductance-van oble resistance combination is, therefore, comesoon to both the anode and grid circuits of devices 2 end S.

The inductance i1 constitutes s by=pess for direct curnent end vsriuble current of low ire= quencies. With such an arrangement, the direct current potential of thecsthodes relative to ground is determined by the snode current drop through resistances 6 end '3. The grid bias of the devices 2 end 3 is, therefore, practically unaffected by the value of resistance 12.

Even order modulation products flow in midi tive sense through the common anode branch of s pus? pull amplifier. Inductance 11 hes a. relatively high impedance to these products so that they flow mainly through resistance 12 and produce drop oi potential therein which is put beck on the crisis by conductor 13.

The inductance 11 also serves to anti-resonate the stray capacities scross the resistance 12, so as to prevent these cepscities from short-cir culture the resistance in the high frequency range. The condensers in shunt to resistonces d and t have low cspscity tothe amplifier en closing cese, whereby the stray capacities across resistance 13 are reduced. Since inductance ii is in effect, c. short-=circuit for direct current around resistance 12, this resistance may be sltered while the smpliiler is in operation with out changing the steady voltsse end current con ditions. It has also been found that the reduc tion of the even order niodulstion products is not tencenesist "ice combination ii and 12 to the grids o2 even or cos 2 and 3, s. voltage due to the .oduls.tion products is impressed on the gzithesthode circuit in the proper phase re== lstionship to greatly reduce the amplitude of rose modulation products.

Resistance 12 is rustle variable to permit sci justice the smount of feedback to the proper vslue tor maximum reduction of the undesired modulation products.

Fig. 2 illustrates the invention as applied to the first end last stages of on odd number of stages oi amplification, in this case three. The genersl arrangement is similar to that of Fig. 1. In the arrangement shown, however, the inductame-resistance combination 14-45 in the lust stage is grounded on the cathode side and a. lead 16, including a variable resistance 17. connects the opposite side of the combination to the midpoint of the secondary winding of input trans- Iormer18 and hence to the grids of space disnee s charge devices 19 and 20. The mid-point of the secondary is connected to one side of inductanceresistsnoe combination 31-22, the other side of which is grounded. The device would operate equsliy as well if each ground were shifted to the opposite side of the corresponding inductones-resistance combinotion.

Three stages A1, A2 and As ereshown, such stsge comprising two space discharge devices connected in push-pull relation. The second stage As is not illustrated in detoil since it may be of ordinary push-pull type.

The inductance-variable resistance combination i l-d5 is similar to the combination 11-12 of Fig. 1. The iced. 18, however, as noted, is taken from that port of the combination removed irom ground.

The operation of the arrangement of Fig. 2 for an odd number of stages is snslogous to that of Fig. 1. Current oi on even order product frequency flowing from anode to cathode in the external circuit causes the ungrounded end of the resistance 15 to assume e. potential which is incpressed on the grids oi devices 19 and 2c. Since the second stage of amplification indicated st its causes e. phase reversal, the voltage led book will be in the proper phase to reduce the even order products. The magnitude of feedbucl: may be regulated by adjusting any or all of the resistunces l5. 1'? end 22.

The arrangement of Fig. 5 is on alternative to that 01 Fig. 2 and may be used where it is desirable to have the center of the grid coil at ground potential. As in Figs. 1 and 2, the even order products current flowing from the anodes 110 to the cathodes in the external circuits of stage A3 causes the ungrounded end of the inductanceresistsnce combination to assume a. potential which is impressed upon the cethode-grid. circuits of the devices in stage A1. Here also the led 115 book voltage is in the proper phcse for degenerstion of even order products.

Cross neutralization is illustrated as applied to stages A1 end As of this figure. It is ordinorily preferred to neutralize the interelectrode 120 cspscities of the tubes in all stages of the verbous modifications, but in order not to unduly con fliise the drawing it is shown only as applied to F s. 3.

It is to be understood that whereas Figs. 2 and 125 3 illustrate s. three-stsge amplifier, the inven tion is applicable to any odd number of stages 718, end that the voltage fed back may be taken from any stage, say the nth, and impressed upon the (re-Olth, (n- 2)th. (rt-shah, etc. stage.

Fig. I} illustrates the invention as applied to on srnplifler having on even number of stages, in this case two. The fedback voltage in this case is obtsined from the opposite end of resistance 14 relative to that in Figs. 2 and 3 and causes that end oi the inductance-resistance combination of stage A2, remote from ground, to assume a. potential with respect to ground opposite in phase to that obtained in Figs. 2 and 3. This potential is impressed on the cathode-grid circuit of the stage A1. The phase of this voltage, which is introduced into the cathode-grid circuits 0! the devices of stage A; is such as to degenerate the current of the even order products.

While in Figs. 3 and 4 the first stage has its 145 own degenerative feedback similar to that oi Fig.

1, and thus is capable of reducing the even order modulation in that stage, the principal compensation for the amplifier as a. whole is effected by the feedback from the last stage, since the am- Leeann plitude of second harmonic in that stage is relatively much larger. The total compensation is, however, a result of both feedbacks in combination, the separate effects of which can be readily controlled by the variable resistances shown.

In each of the Figs. 2, 3 and 4, three variable resistances are shown. In most cases two of these resistances may be fixed and the adjustment made at the third. The three are shown, however, because any one of them may be used.

Throughout this specification, the invention has been spoken of as applied to an even or odd number of stages of amplification. It will be evident that in reality the actual number of stages in the amplifier is not the controlling feature.

but rather the number of stages intervening between the stage from which the degenerating voltage is obtained and the stage to which it is applied. For example, in a five-stage amplifier if the voltage is taken from the fourth stage and fed'back to the first stage the arrangement for an even number of stages (Fig. 4) should be used. Again, in a four-stage system if the voltage is obtained from the third stage and impressed on the first stage, the arrangement for an odd number of stagesv (Figs. 2 or 3) should be used. Generally. however, the degenerating voltage will be obtained from the last stage and impressed on the first stage.

It is desired to point out that the degenerative function of the inductance-resistance combination will be exercised on any currents which may be present in the common branch of the circuits regardless of their source provided'the frequency of such currents is within the received band. However, the amount of fundamental which flows will be small and the eiIect on the funda- .mental output of the amplifier is usually negligible.

Whereas certain modifications of the invention have been illustrated for purposes of description, it is to be understood that the invention is not to be limited to such forms but only within the scope of the appended claims.

What is claimed is:

1. An amplifier comprising a plurality of stages for amplifying a band of frequencies, space discharge devices arranged in push-pull relation in each stage, means in the common branch of the output circuits of one stage across which even order modulation products set up a substantial difference of potential, and means by which this difference of potential is impressed with substantially uniform phase relation over the entire frequency band, on one of the common branches of a preceding stage.

2. An amplifier according to claim 1, in which the means for impressing the difference of potential upon a preceding stage includes an adjustable impedance.

3. An amplifier according to claim 1, in which the means for impressing the difference of poten- 4. An amplifier comprising a plurality of stages, a

space discharge devices arranged in push-pull relation in each stage, an inductance and a resistance in parallel in the common branch of the output circuits of one stage, and means for effectively impressing the potential drop of the even order products of modulation across said resistance upon the input circuit of the space discharge devices of a preceding stage.

5. An amplifier according to claim 4, in which said inductance presents negligible impedance to direct current and currents of low frequencies but a relatively high impedance to currents of the frequency of the band which is to be amplified.

6. Anamplifier according to claim 4, in which the grid-plate capacities of the space discharge devices is neutralized by means of external variable capacities.

'7. In an amplifier, space discharge devices arranged in push-pull relation, input and output circuits for said amplifier, and an inductanceresistance combination in' the common. branch of said output circuit, said inductance providing a by-pass around said resistance for low fre-- quency or direct currents, and means for impressing the drop through said resistance, due to frequencies in the band to be amplified, upon said input circuit.

8. An untuned radio frequency amplifier comprising space discharge devices arranged in pushpuli relation, input and output circuits therefor, a resistance-inductance combination in the common branch of the output circuit and means to adjust the impedance presentedto currents of radio frequency by said resistance-inductance combination.

9. An untuned radio frequency amplifier according to claim 8, in which means are provided for impressing the potential drop across said resistance-inductance combination upon the common branch of a preceding portion of said amplifier.

' 10. In an untuned radio frequency amplifier,

comprising space discharge devices arranged in push-pull relation, the method of neutralizing the incoming even order modulation products of received stations which consists in generating in said amplifier harmonic currents similar to said products and adjusting said harmonic currents as to phase and amplitude so that they balance out in said amplifier circuit the incoming modulation products of the same frequency.

11. An amplifier according to claim 4, in which each stage includes means for neutralizing the internal input-output capacity of said discharge devices whereby said ainplifier is renderedunilateral except for the feedback that impresses said even order modulation products on said in- 

